Wellbores are drilled by a drill bit coupled to the end portion of a drill pipe. The drill bit drills the wellbore to an original “pilot hole” diameter. During or after the drilling of the wellbore to the pilot hole diameter, an underreamer is oftentimes also used to enlarge the diameter of the wellbore. The underreamer is run into the wellbore on the drill pipe in an inactive state. In the inactive state, cutter blocks on the underreamer are folded or retracted inwardly into the body of the underreamer such that the cutter blocks are positioned radially-inward from the surrounding casing or wellbore wall. Once the underreamer reaches the desired depth in the wellbore, the underreamer is actuated to an active state. In the active state, the cutter blocks move radially-outward and into contact with the wellbore wall. The cutter blocks are then used to increase the diameter of the wellbore.
Underreamers are generally spaced axially apart from the drill bit on the drill pipe. For example, the underreamer may be positioned “above” the drill bit by between about 30 m and about 60 m. As such, the underreamer is not able to increase the diameter of this lower portion (30 m-60 m) of the wellbore because the drill bit contacts subterranean formation proximate the base of the wellbore, thereby preventing further downward movement of the underreamer. This portion of the wellbore that remains at the pilot hole diameter is oftentimes referred to as the “rat hole.” What is needed is an improved system and method for increasing the diameter of the rat hole.